1. Field of the Invention
The invention relates generally to devices and methods for electrically insulating and isolating an electromagnetic telemetry system within a drill string. In particular aspects, the invention relates to improved systems for providing an insulative gap assembly within a drill string.
2. Description of the Related Art
Electromagnetic transmitters are used within petrochemical wellbores for the transmission of borehole data and other information to the surface. Such transmitters are often used in measurement-while-drilling (MWD) arrangements wherein downhole conditions are sensed and transmitted to the surface for operators to make adjustments to the drilling operation. An electromagnetic transmitter is formed by electrically separating two metallic drill collars, or subs, by an insulated, tubular “gap sub.” An electrical conductor is disposed through the axial center of the gap sub to permit electrical signals to be alternately provided to the separated drill collars. The separated collars then function as the two poles of a dipole antenna within the earth for sending information wirelessly to a receiver located at the surface of the well.
The use of conventional gap subs has been problematic. Conventional gap sub assemblies have been provided by insulated or non-conductive members that are disposed between two conductive portions in a drill string. The gap sub provides electrical isolation of the drill string portions. In this type of arrangement, a longitudinal conductor must be mechanically supported within and along the length of the gap sub. Depending upon the length of the gap sub, the conductor may have to be more than ten feet in length. Such an arrangement is prone to failure, particularly during drilling when abrasive mud is flowed down through the drill string. Additionally, there are times when the length of the gap sub must be changed in order to alter the characteristic of the transmitter antenna. As a result, the conductor must be exchanged for one of different length as well. This is time consuming and allows for installation errors.
The use of gap subs for electrical isolation is also known. U.S. Pat. No. 5,138,313 issued to Barrington, for example, discloses an electrically insulative gap sub assembly wherein the outer surface of a drill pipe joint is covered by several molded “gap blocks” of insulative material. This technique is expensive and can be complex in construction. In addition, it is prone to damage within the wellbore.
U.S. Pat. No. 4,348,672 issued to Givler describes an insulated drill collar gap sub assembly that is used with a particular toroidal-coupled telemetry system. An insulated gap is formed between a pair of annular sub members by forming a gap between them and filling the gap with a dielectric material. To interconnect the gap sub within the drill string, adjoining sub members are essentially keyed to one another using hexagonal keying. In an alternative version of the device, subs are connected using an axially extending member that resides within an axially extending recess. Pins are used to lock the two subs together, and a dielectric material is disposed in a gap between them. In each case, axial bearing assemblies are necessary to help transmit force through the gap sub. A significant disadvantage to this type of arrangement is the requirement for special tooling to form the various keys or extensions and recesses to mechanically lock the components together. Further, such components would be incompatible with standard drill pipe threaded connections.
There is a need to provide improved methods and devices for integrating a telemetry system into a drill string. It would be an improvement over the prior art to provide simpler construction and cost savings over previous insulative sub constructions. The present invention addresses the problems of the prior art.